The invention concerns the area of electromechanical Microsystems (MEMS or microelectric Microsystems), and more particularly, the micromotors and microgearboxes obtained by deep etching in a substrate of semiconductor material.
Document U.S. Pat. No. 5,631,514 (published on 20 May 1997) describes a micromotor capable of driving a toothed wheel in rotation. The micromotor includes a toothed output element meshing with the toothed wheel, an element forming a connecting rod connected to the output element and interdigital comb-type electrostatic actuators (also known as comb drives) controlling the movement of the element forming a connecting rod. The combined movements of the actuators drive the rotation of the toothed output element.
The various elements making up the micromotor described in this document extend into different planes of the substrate. As a consequence, the manufacture of the micromotor necessitates the etching of a substrate formed from a stack of thin layers and the use of several successive etching masks. Manufacture of the micromotor by means of surface micromachining of the polycrystalline silicon is consequently relatively complex.
Document WO 01/09519 (published on 8 Feb. 2001) describes a MEMS microvalve that includes a shutter element driven in rotation by electrostatic actuators of the comb drive type. The electrostatic actuators act upon the shutter element by friction.
The micromechanism described in the said document does not allow precise control of the positioning or of the speed of the controlled element.
Furthermore, the devices described in documents U.S. Pat. No. 5,631,514 and WO 01/09519 are obtained by surface microetching, that is on thin layers of silicon with a thickness of less than 4 μm. Because of their small thickness, the elements making up these devices are not able to support large deformations. The device structures proposed in those documents are therefore not suitable for the driving of elements with a large driving pitch (10 μm or more), which does not allow one to envisage the coupling of these driving devices with conventional gear trains in areas such as watchmaking.
The document entitled “Single mask, large force, and large displacement electrostatic linear inchworm motors”, Richard YEH, Seth HOLLAR, Kristofer S. J. PISTER (January 2001) describes a micromotor capable of driving a toothed rack device in linear motion. The micromotor is obtained by deep etching in a layer of silicon with a thickness of between 15 and 50 μm. This micromotor is formed from two electrostatic actuators of the comb-drive type connected in series. The combined movements of the actuators drive the rack device in linear motion.
In such an actuator, the movement is obtained by the normal electrostatic force generated between the fingers of the interdigital combs. Now this normal force is dependent on the overlapping area between the fingers and also on the distance between the fingers. The distance between the fingers must be sufficiently low so that the electrostatic force generated is significant. As a consequence, such an actuator necessarily has limited pitch (about 2 μm).
The structure of the actuator proposed in the said document is not suitable for the driving of toothed elements with a pitch of 10 μm or more. Thus, neither does this actuator allow one to envisage the coupling of the driving device with conventional gear trains.